Conventionally, a horn loudspeaker system has been used for increasing the directionality of an amplified sound. Such a conventional sound-amplification apparatus will be described with reference to FIG. 1.
A conventional horn loudspeaker system 20 illustrated in FIG. 1 includes a horn driver 21 and a horn 22 for controlling the acoustic radiation direction and the directionality angle. The horn 22 is an acoustic tube for forwardly radiating an amplified sound by the horn acoustic radiation plane 23. In the figure, i is the diameter of the horn acoustic radiation plane 23, and k is an arrow denoting the direction in which a sound travels through the horn 22.
In order to narrow the directionality angle, it is generally necessary to increase the diameter i of the horn acoustic radiation plane 23. Moreover, in order to reduce the disturbance in the sound pressure frequency characteristic of a sound to be radiated, it is necessary to reduce the frequency change in the acoustic impedance of the horn 22 along the axis thereof. Therefore, in the horn 22 of FIG. 1, the cross section thereof along a direction perpendicular to the sound wave traveling direction k is varied continuously and smoothly. A sound wave reproduced by the horn driver 21 is externally radiated through the horn acoustic radiation plane 23, with its directionality being controlled while it is guided through the horn 22 along the direction of the arrow k.
With the above-described conventional sound-amplification apparatus 20, however, it is necessary to increase the horn acoustic radiation plane 23 in order to obtain a narrow directionality. Moreover, the directional radiation pattern of an amplified sound to be radiated is uniquely determined by the shape of the horn 22. Therefore, it is necessary to replace the horn 22 with another depending upon the required directional radiation pattern.
On the other hand, the reproduction of an acoustic signal should preferably be performed with a desirable S/N ratio even in environmental noise. Therefore, a directional loudspeaker apparatus using an ellipsoidal acoustic reflector has been proposed in the art. Such a conventional example will be described below with reference to figures.
FIG. 2 is a structure diagram illustrating a conventional directional loudspeaker apparatus 30 illustrated in Japanese Laid-Open Publication No. 2-87797.
The directional loudspeaker apparatus 30 includes a concave (parabolic) reflector 31, and a sound source 32 which is provided within the reflector 31 to face a central portion thereof. In this way, a sound output from the sound source 32 is reflected by the reflector 31 so that a sound having a strong directionality along the axis of the reflector 31 is output on the rear side of the sound source 32.
FIG. 3 is a structure diagram illustrating another conventional directional loudspeaker apparatus 40 illustrated in Japanese Laid-Open Publication No. 8-228394.
The directional loudspeaker apparatus 40 includes a concave (hemispherical) reflector 41, and a sound source 42 which is provided within the reflector 41 to face a central portion thereof. The sound source 42 and the reflector 41 are kept at a constant interval, and a rear cover 43 is attached on the rear side of the sound source 42. By covering the rear side of the sound source 42 with the rear cover 43, a rearward sound radiated directly from the sound source 42 is reduced. In this way, the divergent component is reduced, thereby further emphasizing the directional radiation pattern given by the reflected sound from the reflector 41.
In the conventional directional loudspeaker apparatus 30 illustrated in FIG. 2, sound radiation also occurs from the rear side of the sound source 32, whereby the sound is scattered about the sound source 32. Therefore, it is difficult to obtain a narrow directional radiation pattern. In the conventional directional loudspeaker apparatus 40 illustrated in FIG. 3, a rear cover 43 of a sound absorbing material or a sound blocking material is provided in order to reduce the sound radiation from the rear side of the sound source 42. In practice, however, it is difficult to reduce the radiated sound except for very high frequencies.
An on-vehicle sound-amplification apparatus has been one application of such a sound-amplification apparatus. For such a conventional on-vehicle sound-amplification apparatus, a horn loudspeaker system is typically employed in order to efficiently diffuse a reproduced sound to the environment. A conventional on-vehicle sound-amplification apparatus 50 will be described below with reference to FIG. 4.
In FIG. 4, reference numeral 34 denotes a horn driver, 35 a reentrant horn for controlling the acoustic radiation main axis and the directionality angle, 36 a horn acoustic radiation plane, i the diameter of the horn acoustic radiation plane, j the horn length, and k and k′ each denote a horn central axis. Generally, the narrower the directionality angle is, the larger the diameter i of the horn acoustic radiation plane 36 is. In order to obtain a desirable sound pressure frequency characteristic, it is necessary to increase the length of each of the horn central axes k and k′. However, the horn driver 34 and the horn acoustic radiation plane 36 are coupled together with the reentrant horn 35, which is obtained by folding back a horn, so as to reduce the horn length j without reducing the length of the horn central axes k and k′.
In the conventional on-vehicle sound-amplification apparatus 50 having such a structure, a sound wave reproduced by the horn driver 34 is externally radiated through the horn acoustic radiation plane 36, with its directionality being controlled while it is guided through the reentrant horn 35 in the directions indicated by the arrows along the horn central axes k and k′.
In the above-described conventional on-vehicle sound-amplification apparatus 50, it is necessary to increase the horn acoustic radiation plane 36 in order to obtain a narrow directionality. In practice, however, it is difficult to increase the horn acoustic radiation plane 36 because it is provided on the outside of the vehicle body. Therefore, it is difficult to avoid the use of a small-diameter horn loudspeaker system, resulting in a wide directional radiation pattern. Therefore, the radiated sound is transferred to the passengers including the driver, thereby hindering them from having a conversation or listening to the radio.